Microelectromechanical systems (MEMS) refer to a collection of micro-sensors and actuators, which can react to an environmental change under micro-circuit control. The integration of MEMS into traditional radio frequency (RF) circuits has resulted in systems with superior performance levels and lower manufacturing costs. The incorporation of MEMS based fabrication technologies into micro and millimeter wave systems offers viable routes to devices with MEMS actuators, antennas, switches and capacitors. The resultant systems operate with an increased bandwidth and increased radiation efficiency, reduced power consumption, and have considerable scope for implementation within the expanding area of wireless personal communication devices.
MEMS elements comprise a first and a second electrode of which the second electrode is movable to and from the first electrode between an opened position and a closed position, in which opened position there is an airgap between the first and the second electrode. A dielectric layer may be present on top of the first electrode. This leads to the fact that the first electrode does not make electrical contact with the second electrode in its closed position, but forms a capacitor therewith. The other electrode or electrodes may also be provided with dielectric layers or native oxides if so desired.
The fact that the second electrode must be movable, but still be incorporated in a mechanically stable construction results in the fact that the devices are usually provided with a mechanical layer of sufficient thickness and mechanical stability. The second, movable electrode may be provided in the mechanical layer, but this is not necessary. It may also be provided in an additional intermediate layer between the beam and the first electrode. In particular, recent experiments have shown that it is advantageous to fabricate a MEMS element with a third electrode in the mechanical layer, in addition to the second electrode that is present in an intermediate electrode layer. The second electrode is then movable not only towards the first electrode, but also towards the third electrode.
Surface micro-machining is a common method for the fabrication of MEMS, and the processing sequence used in surface micro-machining is illustrated schematically in FIG. 1 of the drawings. A base layer 10, a sacrificial layer 16 and a beam or mechanical layer 12 are deposited on a substrate 14 and structured. The beam layer 12 is made free-standing by etching the sacrificial layer 16. This means that the beam layer has a larger area that is not supported by a substrate. The beam layer is then supported through supports to one or more contact pads in the base layer. This support may be present under the beam layer, but also more or less laterally to the beam layer. The latter alternative is particularly present if the beam layer has a bridge-like or membrane-like construction, and provides a better elasticity for vertical movement to the beam layer. The beam layer may be electrically conducting as in this case, in which the contact pads are also electrical contact pads. However, this is not necessary. In this embodiment, the second electrode is defined in the mechanical layer 12, and the first electrode is defined in the base layer.
The critical processing step in the above-mentioned sequence is the etching of the sacrificial layer, and the etchant for etching this layer should ideally fulfill several criteria:
it should not etch the beam and base layers;
it should not lead to sticking of the second electrode to the substrate after etching; and
it should result in the fact that the second electrode is movable between its ultimate position without any mechanical resistance from parts of the sacrificial layer that are not removed.
Several MEMS systems of materials with their sacrificial layer etchant are known. The most common system is the wet-chemical etching of a SiO2 sacrificial layer in a solution of HF using beam and base layers (12, 10) consisting of Si. However, the main disadvantage of the known system is that the free-standing layer 12 tends to stick to the base layer as a result of capillary forces during drying of the substrate after HF etching. Another type of system, which does not have this drawback is one in which the etchant consists of a gas or plasma rather than a liquid. This type of system is known as dry chemical etching. A known system of this type uses an O2 plasma for sacrificial layer etching. In this case, the sacrificial layer consists of a polymer and the base layer and mechanical layers comprise a metal. The substrate, and any dielectric layers on the substrate or on the base layer can be exposed additionally.
However, a drawback of this system is the fact that the polymer sacrificial layer limits the processing freedom of the beam layer. The processing temperature of the beam layer is then limited, since polymers tend to flow and/or outgas at high temperatures (200-300° C.). This considerably restricts the choice of material for the mechanical layer. Furthermore, the use of polymers for structural layers is non-standard in common IC processes.
The restriction is particularly problematic if the sacrificial layers must be removed only locally. Suitably, the mechanical layer is also used as interconnects, and possibly other elements such as inductors and capacitors are present in the device. They are laterally displaced from the MEMS element, but defined in the same layers. The mechanical layer has therein a function of interconnect layer, and the interconnects or coils that have a size in the order of micrometers or even millimeters ought not to hang in the air without sufficient support. In order to remove the sacrificial layer only locally, parts thereon must be protected by a protective layer, such as a photoresist. A photoresist, is however, a polymer layer and the etchant that removes a polymer sacrificial layer tends to remove the polymer layer as well.